TODO for custom degree shift of d_r

[dalcde]

The intended application of this is to use ext-websocket to work with other spectral sequences, which may have different degree shifts.

Turns out the previous commit wasn't quite enough:

• Implement this for the JS interface as well.
• add_product_differential currently automatically infers the page from the coordinates, which doesn't work in general.
• assert_eq in add_differential still hard codes the shift